1. Field of the Invention
The present invention relates to an image sensing apparatus for sensing a still image and/or moving image.
2. Description of the Related Art
Conventionally, image sensing apparatuses such as electronic cameras and the like, which record/play back still images and moving images sensed by a solid-state image sensing element such as a CCD or the like using memory cards having solid-state memory elements as recording media, have already been commercially available.
Furthermore, another electronic still camera senses images by selectively using one of an aperture (Av) priority image sensing mode for automatically increasing/decreasing the shutter speed to obtain an appropriate exposure value in correspondence with an arbitrarily selected aperture value, and a shutter speed (Tv) priority mode for automatically increasing/decreasing the aperture value to obtain an appropriate exposure value in correspondence with an arbitrarily selected shutter speed.
Moreover, still another electronic still camera comprises an AEB (auto exposure bracketing) image sensing mode that automatically brackets three frames with a standard exposure value, overexposure value, and underexposure value during single image sensing operation. Such cameras enable the photographer to easily obtain an appropriate exposure value upon image sensing under complicated light conditions or upon image sensing using an electronic camera that comprises an image sensing element with a narrow latitude.
Upon image sensing using a solid-state image sensing element such as a CCD or the like, a dark noise correction process is done by computations using dark image data which is read out after charge accumulation in the same manner as in actual image sensing while the image sensing element is not exposed, and actually sensed image data which is read out after charge accumulation while the image sensing element is exposed.
With this process, sensed image data can be corrected for image quality deterioration such as pixel omission or the like caused by dark current noise produced by the image sensing element and small scratches unique to the image sensing element, thus obtaining high-quality sensed image data.
Especially, since dark current noise increases with increasing charge accumulation time and temperature rise of the image sensing element, if exposure for a long period of time (seconds) or at high temperature is done, a great image quality improvement effect can be obtained, and the dark noise correction process is a function useful for the electronic camera users.
In this manner, since dark current noise increases with increasing charge accumulation time and temperature rise of the image sensing element, the charge accumulation time of the image sensing element for capturing the dark image data must be determined in correspondence with the shutter speed (Tv value) obtained by exposure measurement.
On the other hand, when AEB image sensing is done in the aperture (Av) priority image sensing mode, the shutter speed must be changed for each frame upon automatic bracketing for three frames.
Therefore, in an image sensing apparatus such as an electronic camera, when the shutter speed has changed, a new charge accumulation time is set. When the charge accumulation time has been changed, the dark image must be re-captured. That is, upon executing automatic sequential exposure image sensing for three frames, the dark image must be re-captured every actual image sensing.
For this reason, when continuous-shot AEB image sensing is done in the aperture (Av) priority image sensing mode, the dark image must be captured three times upon sensing each of frames to be continuously sensed, and the image sensing interval for each frame upon continuous shot image sensing is prolonged by the dark image sensing time, and the shutter release time lag becomes longer by the dark image sensing time, thus making the user miss a shutter chance or reducing the continuous shot frame speed.